This invention relates to an icebreaking ship.
A conventional icebreaker is a special purpose vessel the design of which is made with emphasis on its icebreaking characteristics. The operating season of an icebreaker is normally only a few months in each year, which means that during the greater part of the year the money invested in an icebreaker is non-performing. Hence, there is a need for developing the conventional icebreaker so that without impairing its icebreaking characteristics it should also be able to serve another purpose, for instance, as an off-shore supply vessel, a diving support vessel, a towing vessel and/or a research vessel.
The bow form of an icebreaker in accordance with contemporary designs is not sharp, as in a ship designed only for navigation in open water. In accordance with contemporary designs, the bow form of an icebreaker, when viewed in horizontal section in the vicinity of the design waterline, is either rounded with a large radius of curvature or is completely flat over a large part of the width of the ship. See, for example, Canadian Patent 1,026,160. An extreme example of this type of design is the Swedish icebreaker ODEN, which was built in 1988. The bow of the ODEN, when viewed in horizontal section, has a flat central portion nearly 20 m wide. Such an extremely flattened bow is designed for difficult ice conditions, and is not favorable for operation in open water. In particular, the resistance to movement in open water is high and wave impact blows are very strong.
Most conventional ships employ pushing screws, i.e. screws that are aft of the shaft that delivers torque to the screw and transmits thrust from the screw to the hull of the ship, as propulsion means It is also known to employ pulling screws as propulsion means for a ship. For example, Finnish examined patent publication 76977 discloses a ship that is provided in its aft part with a pulling screw. The propulsion force provided by a pulling screw is transmitted to the hull of the ship by a shaft that is in tension.
Icebreakers have been constructed with two forwardly projecting pulling bow screws that give about 40% of the screw thrust of the vessel. In such a vessel, the bow screws produce water streams that function as a lubricant between the hull and the surrounding ice.
A ship's rudder causes a ship to turn by forcing the aft part of the ship sideways. When a conventional icebreaker is moving in the forward direction along a channel in an ice field, the edges of the ice field resist sideways movement of the aft part of the icebreaker, impairing the maneuverability of the ship.